A conventional insole, in general, consists of a body made of materials such as synthetic resin foam or elastomer covered with leather, woven fabrics, unwoven fabrics or the like adhesively bonded thereto. There are some types of such insoles which include one having a flat shape cut out of a sheet material in a shape of the foot, one having a raised portion in the area corresponding to the arch portion of the foot, and one having a raised portion surrounding the heel of the foot so as to fit thereto.
These insoles are employed for the purpose of improving the fitness of the foot, especially the heel of the foot within a shoe by filling the space between the foot and the interior of the shoe, thereby improving the stability of the heel of the foot while walking or running.
The most common type of resin foam utilized for the insoles is the one which can be deformed by compression applied by the heel of the wearer, so as to conform to the shape of the heel, and bounces back to the original shape when the wearer takes off the shoe as described in, for example, Japanese utility model public disclosure Sho 59-42892 and Japan utility model public disclosure Sho 59-23528. There is another type of resin foam which does not bounce back to the original shape once compressively deformed even when the wearer takes off the shoe as described in, for example, Japan utility model public disclosure Sho 61-16807 and Japan utility model public disclosure Sho 62-28163. Furthermore, there is another type, disclosed in Japan utility model public disclosure Sho 62-7126, which is formed by mixing uncured synthetic foam with curing agent, pouring the mixture into a bag-like sheet, and placing the foot of the wearer on the mixture filled bag to press the shape of the foot thereto, whereby an insole which has a close fitness to the foot is formed when the curing process is completed.
Most of these conventional insoles utilize an impact cushioning layer or space filler made of a synthetic foam such as polyurethane, polyolefin, or the like utilizing its physical property of compression-deformation.
Therefore, when a wearer puts on a shoe having such an insole, the impact cushioning layer of the insole made of synthetic foam is deformed due to the weight of the wearer to comform to the shape of the foot of the wearer.
The impact cushioning layer absorbs the impact force applied thereto from the ground during walking and running. However, the impact cushioning layer formed from the synthetic foam is easily deformed by the pressure applied by the heel and can not prevent rolling of the heel.
Therefore, the heel is unstable during walking and running, resulting in rolling of the ankle, which may cause excessive pronation and supination of the foot that can hurt the foot of the-wearer.
In addition, the commonly used materials used to form the impact cushioning layer or the space filler of the insoles such as polyolefin resin foam and chloroprene rubber foam often exhibit so-called permanent set due to the compressive force applied through wearing the shoe repeatedly. That can cause deterioration in its flexibility and the ability to restore the original shape and, therefore, it will no longer function as the impact cushioning layer or the space filler.